1. Field of the Invention
This invention relates to an image sensing device suited for obtaining an accurate black level.
2. Description of the Prior Art
Previously, in an apparatus such as a video camera of the type using a solid-state image sensor, a light shielded part was arranged at the image sensing cells corresponding to the back porch of the horizontal retrace line period. This was done for the purpose of obtaining a stable optical black reference signal during each horizontal retrace line period in clamping a signal read out. The reason for this arrangement was because the dark current which caused the black level fluctuations of the solid-state image sensor increased about twofold every time the temperature of the sensor rose 8.degree. C. Further, the temperature range in which video cameras are used varies from -10.degree. C. to +40.degree. C. Therefore, several to scores of image sensing cells corresponding to back porches of the horizontal retrace lines within the solid-state image sensor are generally shielded from light to have dark currents solely produced there. The signal level of this part is then clamped at a reference black level. In order to have the clamping operation performed in a stable manner, it is preferable to allocate a large number of image sensing cells to the light shielded part. However, use of many image sensing cells for the light shielded part results in an insufficient number of image sensing cells in the horizontal direction. This in turn degrades resolution. It is difficult to increase the number of image sensing cells within a limited space unless the area and the integration degree of the image sensor are increased by improvement in process technology.
To solve this problem, a feedback type clamping method has generally been employed for a video camera using a camera tube. Consistent with this method, a system is arranged to include a clamping circuit which clamps the level of the beam blanking period at a reference level, i.e., black level. A processing circuit which amplifies the clamped signal and gamma corrects it. To stabilize the output of this system, the output of the processing circuit is compared with a predetermined reference level and a feedback process is arranged to negate any difference detected by this comparison. However, when this method is applied to a solid-state image sensing device, the device tends to be affected by a noise produced during the horizontal blanking period, because a solid-state image sensing device generally has a great deal of noise during the blanking period. In addition, a resetting action performed on the reference potential at the time of signal reading causes variations of the saturation resistance of a switching transistor. Further, the reset potential varies with the temperature characteristic. Therefore, it is impossible to include the blanking period as it exists in the signal to be clamped at the time such clamping occurs.